Mex, the two hour tip is very useful -- I had been waiting just an hour after disconnecting the battery to take the measurement.
So are you saying that an ordinary hydrometer, which costs less than ten dollars US, can give an accurate state of charge? The thing looks so amateurish -- the plastic arrow wobbles around and eventually points to a color zone -- green for good, yellow for fair, red for bad. That can't be what you have in mind, is it??
And concerning the other comments about "does it give me a day's worth of power" -- yes, it does. But I want to know for sure how much reserve I really have. For one thing, if I have a lot of extra capacity, I can stop nagging my poor wife to turn off unnecessary lights!
Also, if I can develop a good (but not prohibitively complicated) annual test, I can see how my batteries are aging. That will let me know two things -- when will they need replacing? Is my usual battery maintenance technique really adequate? (I keep them on a Battery Minder Plus -- I know the blinking light is a hoax, but it makes me feel better!!) ;)
